Memory is one type of integrated circuitry, and is used in computer systems for storing data. Integrated memory is usually fabricated in one or more arrays of individual memory cells. The memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information.
One type of memory is phase change random access memory (PCRAM). Such memory utilizes phase change material as a programmable material. Example phase change materials that may be utilized in PCRAM are ovonic materials, such as various chalcogenides.
The phase change materials reversibly transform from one phase to another through application of appropriate electrical stimulus. Each phase may be utilized as a memory state, and thus an individual PCRAM cell may have two selectable memory states that correspond to two inducible phases of the phase change material.
A PCRAM cell may comprise a volume of phase change material between a pair of electrodes. A portion of the volume will change phase during operation of the cell, and such portion may be referred to as a switching volume. The switching volume is often a small fraction of the overall volume of the phase change material, and thus the majority of the phase change material within a memory cell may remain in a static phase during operation of the cell.
FIG. 1 shows a prior art memory cell 10 comprising a phase change material 14 between a pair of electrodes 12 and 16. The phase change material has a switching volume 18 therein, and such switching volume is directly over and against the bottom electrode 12. An outer boundary of the switching volume is diagrammatically illustrated with a dashed line 19.
In operation, the bottom electrode may function as a heater to elevate a temperature within the switching volume which, in combination with self-heating within the phase change material, may induce a phase change. A region 21 corresponds to a part of the switching volume that is directly against the bottom electrode. Such region may be the highest temperature region of the switching volume material during operation of the memory cell in ideal prior art situations in which heat is not lost through the bottom electrode.
A problem with the configuration of FIG. 1 is that there are may be heat loss from the switching volume through the bottom electrode. Such heat loss reduces operational efficiency of the memory cell. Another problem is that the highest temperature region of the switching volume may be shifted away from the bottom electrode due to heat loss through the electrode, which can lead to higher temperature requirements and programming current requirements. Some prior art constructions may have a highest temperature region of the phase change material which is much hotter than a melting point of the phase change material, which may be detrimental to the memory cell over time and/or may lead to excessive power consumption. Also, the prior art memory configuration of FIG. 1 may require a large switching volume cross-sectional area to fully cover an electrode surface, which may lead to high programming current requirements. It would be desirable to develop new memory cells that alleviate or prevent the problems associated the prior art memory cell of FIG. 1.